The present invention relates to electrostatographic imaging and recording apparatus, and to assemblies in these apparatus for fixing toner to the substrates. The present invention relates particularly to fuser members, and fusing surface layers for fuser members, in the toner fixing assemblies.
Generally in electrostatographic reproduction, the original to be copied is rendered in the form of a latent electrostatic image on a photosensitive member. This latent image is made visible by the application of electrically charged toner.
The toner forming the image is transferred to a substrate, also referred to in the art as a “receiver”, such as paper or transparent film, and fixed or fused to the substrate. Where heat softenable toners, for example, thermoplastic polymeric binders, are employed, the usual method of fixing the toner to the substrate involves applying heat to the toner, once it is on the substrate surface, to soften it, and then allowing or causing the toner to cool. This application of heat in the fusing process is preferably at a temperature of about 90° C.-220° C.; pressure may be employed in conjunction with the heat.
A system or assembly for providing the requisite heat and pressure is generally provided as a fusing subsystem, and customarily includes a fuser member and a support member. The various members that comprise the fusing subsystem are considered to be fusing members; of these, the fuser member is the particular member that contacts the toner to be fused by the fusing subsystem. The heat energy employed in the fusing process generally is transmitted to toner on the substrate by the fuser member. Specifically, the fuser member is heated; to transfer heat energy to toner situated on a surface of the substrate, the fuser member contacts this toner, and correspondingly also can contact this surface of the substrate itself. The support member contacts an opposing surface of the substrate.
Accordingly, the substrate can be situated or positioned between the fuser and support members, so that these members can act together on the substrate to provide the requisite pressure in the fusing process. In cooperating, preferably the fuser and support members define a nip, or contact arc, through which the substrate passes. Also as a matter of preference, the fuser and support members are in the form of fuser and pressure rollers, respectively. Yet additionally as a matter of preference, one or both of the fuser and support members have a soft layer that increases the nip, to effect better transfer of heat to fuse the toner.
During the fusing process toner can be offset from the substrate to the fuser member. Toner transferred to the fuser member in turn may be passed on to other members in the electrostatographic apparatus, or to subsequent substrates subjected to fusing.
Toner on the fusing member therefore can interfere with the operation of the electrostatographic apparatus and with the quality of the ultimate product of the electrostatographic process. This offset toner is regarded as contamination of the fuser member. Therefore, improving the release of the fuser member fusing surface layer, and thereby preventing or at least minimizing this contamination, is a desirable objective.
A factor in achieving sufficient fusing quality is providing sufficient heat transfer from the fusing surface layer of the fuser member to the substrate toner. This heat transfer is improved by increasing the fusing surface layer's thermal conductivity, which in turn is increased by incorporating thermally conductive filler in this layer. Particularly, high speed fusing of thermoplastic toners can require the presence of thermally conductive filler in the fusing surface layer, in order to increase the thermal conductivity of this layer.
Unfortunately, thermally conductive fillers are characterized by high surface energy; because of this property they serve as sites for toner to adhere to. These sites remove toner from the substrate and the displaced toner contaminates the fuser member surface. Polyester toners in particular are especially prone to interacting with high energy sites in this manner to cause such contamination. Moreover, increasing the thermally conductive filler content of the fusing surface layer, by providing more reactive sites for the toner, increases toner offset, and also increases contamination of the fuser member.
It would therefore be desirable, where thermally conductive filler particles are present in the fusing surface layer, to lessen, and ideally to minimize, the amount of the filler necessary to achieve a desired thermal conductivity, while also lessening, and ideally minimizing, the number of toner reactive sites in the fusing surface layer, and lessening toner contamination.
Particularly, changes as to the type, particle shape, and orientation of thermally conductive filler have been employed, for the purpose of optimizing thermal conductivity while minimizing toner contamination resulting from the filler's presence. In this regard, different methods for trying to increase thermal conductivity, without increasing the proportion by volume of thermally conductive filler, are known in the art. In one such method, the thermally conductive filler is provided in the form of high aspect ratio particles e.g., fibers, needles, and other elongated shapes.
However, one problem has been that such fillers lead to high viscosities. Another is that the high aspect ratio particles are less effective for conducting heat unless they are advantageously oriented.
Specifically, elongated particles are more efficient for conducting heat in the proper direction if they are at right angles to the fuser base, radially aligned, if the fuser base is a cylindrical core, belt on rollers, or a core-mounted plate. However, they are less efficient if they are positioned parallel to the core, axially aligned, if the fuser base is a core, a belt, or is core mounted as indicated.
It is difficult to provide a fusing surface layer with the desired perpendicular (radial) alignment of high aspect ratio thermally conductive filler. Application to the fuser base of the fusing surface layer coating material tends to leave too high a proportion of high aspect ratio filler in parallel (axial) alignment. Attempts have been made to rectify this disadvantageous positioning of high aspect ratio filler by magnetic alignment. However, this is a complicated process; and there are few fillers that respond sufficiently to magnetic fields. Accordingly, magnetic alignment has not been a very satisfactory solution to the difficulties inherent to using high aspect ratio thermally conductive filler.
With respect to the foregoing, it would therefore be desirable if thermal conductivity could be increased, with concurrent minimization of toner contamination, independently of the filler particle type, or shape, or orientation.
Still further, it would be desirable to increase the thermal conductivity of the fusing surface layer without changing the layer elastomer composition.